Rollup merge of rust-lang#54958 - RalfJung:static-assert, r=oli-obk

add a macro for static (compile-time) assertions

Cc @oli-obk

src/librustc/macros.rs

@@ -62,6 +62,16 @@ macro_rules! span_bug {
     })
 }
 
+#[macro_export]
+macro_rules! static_assert {
+    ($name:ident: $test:expr) => {
+        // Use the bool to access an array such that if the bool is false, the access
+        // is out-of-bounds.
+        #[allow(dead_code)]
+        static $name: () = [()][!$test as usize];
+    }
+}
+
 #[macro_export]
 macro_rules! __impl_stable_hash_field {
     ($field:ident, $ctx:expr, $hasher:expr) => ($field.hash_stable($ctx, $hasher));

src/librustc/middle/region.rs

@@ -167,8 +167,7 @@ newtype_index! {
 impl_stable_hash_for!(struct ::middle::region::FirstStatementIndex { private });
 
 // compilation error if size of `ScopeData` is not the same as a `u32`
-#[allow(dead_code)]
-static ASSERT: () = [()][!(mem::size_of::<ScopeData>() == 4) as usize];
+static_assert!(ASSERT_SCOPE_DATA: mem::size_of::<ScopeData>() == 4);
 
 impl Scope {
     /// Returns a item-local id associated with this scope.

src/librustc/ty/context.rs

@@ -829,12 +829,9 @@ impl<'tcx> CommonTypes<'tcx> {
     fn new(interners: &CtxtInterners<'tcx>) -> CommonTypes<'tcx> {
         // Ensure our type representation does not grow
         #[cfg(target_pointer_width = "64")]
-        #[allow(dead_code)]
-        static ASSERT_TY_KIND: () =
-            [()][!(::std::mem::size_of::<ty::TyKind<'_>>() <= 24) as usize];
+        static_assert!(ASSERT_TY_KIND: ::std::mem::size_of::<ty::TyKind<'_>>() <= 24);
         #[cfg(target_pointer_width = "64")]
-        #[allow(dead_code)]
-        static ASSERT_TYS: () = [()][!(::std::mem::size_of::<ty::TyS<'_>>() <= 32) as usize];
+        static_assert!(ASSERT_TYS: ::std::mem::size_of::<ty::TyS<'_>>() <= 32);
 
         let mk = |sty| CtxtInterners::intern_ty(interners, interners, sty);
         let mk_region = |r| {